DNA repair is a mechanism of resistance to antitumor DNA damaging agents and to radiotherapy. Rosen EM et al., Cancer Invest. 17(1): 56–72 (1999). The ability of cancer cells to recognize and repair DNA damage inflicted by cancer therapy is an important mechanism of resistance to treatment. Thus, inhibition of DNA repair is a key strategy in enabling cancer therapy.
Activation of the poly(ADP-ribose) polymerase-1 (PARP-1) enzyme is an immediate cellular response to genotoxic stress and is part of a genomic surveillance mechanism that responds to DNA damage, triggering signaling events that can lead to cellular recovery. PARP-1 is specifically activated by binding to DNA strand breaks. PARP-1 has been shown to be a target for the development of radio and chemo sensitizing agents in cancer treatment as well as providing protection from stroke. Szabo C & Dawson V L, Trends Pharmacol Sci. 19(7): 287–98 (1998). Current inhibitors target a conserved catalytic domain of poly(ADP-Ribose) polymerase-1 (PARP-1) present in all of the PARP family members. Ruf A. et al., Biochemistry 37(11): 3893–900 (1998); Tentori L et al., Pharmacol Res. 45(2): 73–85 (2002); Jacobson M K & Jacobson E L, Trends Biochem Sci. 24(11): 415–7 (1999).
Knockout experiments have shown that the therapeutic benefits of PARP-1 inhibition are a direct result of the selective inhibition of PARP-1. Shall S & de Murcia G, Mutat. Res. 4601: 1–15 (2000). Although PARP-1 knockout is not lethal, it leads to genomic instability and enhances the cytotoxicity of DNA damaging agents used in cancer therapy. Several PARP-1 inhibitors are currently in preclinical development for cancer therapy. Each of these inhibitors targets the binding site of the required substrate of the enzyme, nicotinamide adenine dinucleotide (NAD). White A W et al., J. Med. Chem. 43: 4084–97 (2000).
Until recently, PARP-1 was the only known enzyme with ADP-ribose polymerizing activity. PARP-1 has now been found to be one of a family of enzymes with PARP activity. Jacobson M K & Jacobson E L, Trends Biochem. Sci. 24: 415–7 (1999). Amino acid sequence comparisons of the members of the PARP family indicate that there is similarity in their NAD binding sites (pADPRT domain, see, FIG. 1A). Thus, the current inhibitors lack selectivity, because they target an NAD binding site common to all PARP family members.
A double knockout of PARP-1 and PARP-2 results in an embryonic lethal. Schreiber V. et al., J. Biol. Chem. (2002). PARP-1 inhibitors have also inhibit PARP-2, thus it is likely that the current strategy of inhibitor design may lead to toxic effects. Perkins E. et al., Cancer Res. 61(10): 4175–83 (2001).
Thus, while PARP-1 remains a promising therapeutic target, the discovery of multiple PARPs raises questions of inhibitor selectivity not heretofore considered.